Method for electrolysis of water and method for preparing catalysts for electrolysis of water

ABSTRACT

A method for electrolysis of water and a method for preparing a catalyst for electrolysis of water are provided. The method for electrolysis of water includes using a high entropy alloy as a catalyst. Further, the method for preparing a catalyst for electrolysis of water includes the steps of placing a substrate in an aqueous electrolyte containing a high entropy alloy precursor and performing an electroplating process on the substrate to form a high entropy alloy catalyst on the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 109100708, filed on Jan. 9, 2020. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for electrolysis of water and a methodfor preparing catalysts for electrolysis of water.

2. Description of Related Art

In the present chemical industry, synthesis of hydrocarbon compoundsoften requires hydrogen to participate in the relevant reaction, andfossil fuel is generally used as a raw material for hydrogen production.The present most popular and economical commercial process for H₂production is steam-methane reforming, which uses fossil fuels as theraw material and generates CO₂ as the by-product. It is definitely anenvironmental unfriendly and a non-sustainable H₂ production process,and development of green H₂ production is in urgent need.

SUMMARY OF THE INVENTION

The invention provides a method for electrolysis of water. According tothe method, a high entropy alloy is used as a catalyst.

The invention provides a method for preparing catalysts for electrolysisof water. According to the method, an electroplating process isperformed to form a high entropy alloy catalyst on a substrate.

According to the method of the invention for electrolysis of water, thehigh entropy alloy is used as the catalyst.

In an embodiment of the method for electrolysis of water of theinvention, the high entropy alloy is, for example, composed of iron,cobalt, nickel, copper and molybdenum. On the basis of a total molenumber of the high entropy alloy, a content of each of iron, cobalt,nickel, copper and molybdenum is, for example, in a range of 5 to 35 at.%.

In an embodiment of the method for electrolysis of water of theinvention, the high entropy alloy is, for example, composed of iron,cobalt, nickel, copper, molybdenum and manganese. On the basis of atotal mole number of the high entropy alloy, a content of each of iron,cobalt, nickel, copper, molybdenum and manganese is, for example, in arange of 5 to 35 at. %.

In an embodiment of the method for electrolysis of water of theinvention, for example, an aqueous solution with a pH value of 7 to 14is used as an electrolyte for application of an anode.

In an embodiment of the method for electrolysis of water of theinvention, for example, an aqueous solution with a pH value of 0 to 14is used as an electrolyte for application of a cathode.

The method for preparing a catalyst for electrolysis of water of theinvention includes the following steps. A substrate is placed in anaqueous electrolyte containing a high entropy alloy precursor. Anelectroplating process is performed on the substrate to form a highentropy alloy catalyst on the substrate.

In an embodiment of the method for preparing a catalyst for electrolysisof water of the invention, the high entropy alloy precursor is, forexample, composed of ferric chloride, cobalt chloride, nickel chloride,cupric chloride and ammonium molybdate.

In an embodiment of the method for preparing a catalyst for electrolysisof water of the invention, the high entropy alloy precursor is, forexample, composed of manganese chloride, ferric chloride, cobaltchloride, nickel chloride, cupric chloride and ammonium molybdate.

In an embodiment of the method for preparing a catalyst for electrolysisof water of the invention, a current density of the electroplatingprocess is, for example, in a range of 2 to 6 A/cm².

In an embodiment of the method for preparing a catalyst for electrolysisof water of the invention, the substrate is, for example, a poroussubstrate.

Based on the above, the high entropy alloy is used as the catalyst forelectrolysis of water in the invention, so that overpotential requiredby water electrolysis can be effectively reduced, and electric energyconsumption is substantially reduced. Additionally, in the invention,the high entropy alloy catalyst is formed by the electroplating process,so that preparation steps of the high entropy alloy catalyst can besimplified, and preparation cost can be reduced. The formed high entropyalloy catalyst may have a high surface area, and water electrolysisefficiency can be effectively improved.

To make the foregoing features and advantages of the present disclosuremore comprehensible, a detailed description is made below with referenceto the accompanying drawings by using embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a method for electrolysis of water drawnaccording to an embodiment of the invention.

FIG. 2 is a flow diagram of a method for preparing a high entropy alloycatalyst drawn according to an embodiment of the invention.

FIG. 3 is an SEM (Scanning Electron Microscope) image of a five-elementhigh entropy alloy catalyst of an experimental example 3.

FIG. 4 is a TEM (Transmission Electron Microscope) image of thefive-element high entropy alloy catalyst of the experimental example 3.

FIG. 5 is an SEM image of a six-element high entropy alloy catalyst ofan experimental example 4.

FIG. 6 is a TEM image of the six-element high entropy alloy catalyst ofthe experimental example 4.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, a high entropy alloy generally refers to a five-elementalloy, a six-element alloy or multi-element (more than six elements)alloy, and a content of each metal element is in a range of 5 to 35 at.%. That is, these metal elements are all main components of the highentropy alloy, and the high entropy alloy may also include other traceelements.

In the invention, the high entropy alloy is used as a catalyst forelectrolysis of water. Overpotential required by water electrolysis canbe reduced, so that electric energy consumption is reduced.Additionally, fossil fuel is not needed to be used as a hydrogenproduction raw material, so that emission of carbon dioxide can beeffectively reduced.

Additionally, in the invention, the above high entropy alloy catalyst isformed on a substrate through an electroplating process, so thatpreparation steps of the high entropy alloy catalyst can be simplified,and preparation cost can be reduced. In addition, the high entropy alloycatalyst is formed by the electroplating process, so that the highentropy alloy catalyst can have a three-dimensional structure and thushave a high surface area, and water electrolysis efficiency can beeffectively improved.

A method for electrolysis of water and a method for preparing catalystsof the invention are respectively illustrated hereafter.

FIG. 1 is a flow diagram of a method for electrolysis of water drawnaccording to an embodiment of the invention. Referring to FIG. 1, in astep 100, an electrode with a high entropy alloy catalyst formed on thesurface is inserted into an electrolyte. In the present embodiment, thefive-element high entropy alloy and the six-element high entropy alloyare respectively used as a catalyst for electrolysis of water, but theinvention is not limited thereto. In other embodiments, themulti-element (more than six elements) high entropy alloy can be used asthe catalyst for electrolysis of water.

Under the condition of using the five-element high entropy alloy as thecatalyst for electrolysis of water, the high entropy alloy includingiron, cobalt, nickel, copper and molybdenum as main components can beused, but the invention is not limited thereto. On the basis of a totalmole number of the five-element high entropy alloy, a content of each ofiron, cobalt, nickel, copper and molybdenum is in a range of 5 to 35 at.%. Preferably, iron, cobalt, nickel, copper and molybdenum exist in thefive-element high entropy alloy according to a molar proportion of1:1:1:1:1. Under this condition, the five-element high entropy alloycatalyst does not include a noble metal, so that production cost can bereduced, and commercialization is facilitated.

Under the condition of using the six-element high entropy alloy as thecatalyst for electrolysis of water, the high entropy alloy includingmanganese, iron, cobalt, nickel, copper and molybdenum as maincomponents can be used, but the invention is not limited thereto. On thebasis of a total mole number of the six-element high entropy alloy, acontent of each of manganese, iron, cobalt, nickel, copper andmolybdenum is in a range of 5 to 35 at. %. Preferably, manganese, iron,cobalt, nickel, copper and molybdenum exist in the six-element highentropy alloy according to a molar proportion of 1:1:1:1:1:1. Under thiscondition, the six-element high entropy alloy catalyst does not includea noble metal, so that production cost can be reduced, andcommercialization is facilitated.

Additionally, in the present embodiment, an anode and a cathode arerespectively inserted into different electrolytes. In detail, the anodeis inserted into an aqueous solution with a pH value of 7 to 14, and thecathode is inserted into an aqueous solution with a pH value of 0 to 14.However, the invention is not limited thereto. In other embodiments, theanode and the cathode are inserted into an aqueous solution with the pHvalue of 7 to 14.

In a step 102, a voltage is applied onto the anode and the cathode, sothat an oxidation reaction occurs at the anode, and a reduction reactionoccurs at the cathode. At the moment, oxygen gas is produced at theanode, and hydrogen gas is produced at the cathode. With the presentmethod, fossil fuel is not needed to be used as a hydrogen productionraw material. Therefore, carbon emission caused by generation of carbondioxide can be avoided. Additionally, with the high entropy alloycatalyst, the overpotential required by water electrolysis can beeffectively reduced, and an effect of reducing the electric energyconsumption is achieved.

The effects brought by the present embodiment will be illustratedhereafter by experimental examples.

Experimental Example 1

The five-element high entropy alloy including iron, cobalt, nickel,copper and molybdenum as main components is used as the catalyst, and 1M of potassium hydroxide aqueous solution is used as the electrolyte forperforming water electrolysis.

A three-electrode system is used, a CV (cyclic voltammetry) method isused to confirm redox peak positions before electrochemical polarizationcurve measurements of HER (hydrogen evolution reaction) and OER (oxygenevolution reaction), and meanwhile, the system is enabled to reach astable state at the same time to perform subsequent experimental steps.

For OER and HER measurements in alkaline media, a CV scanning range is 0to 1 V (vs. Hg/HgO electrode) and −0.8 to −1.5 V (vs. Hg/HgO electrode),respectively. For HER measurements in acidic media, the CV scanningrange is −0.5 to −1.5 V (vs. Hg/HgO electrode), a scan rate is 100 mV/s,a scanning cycle number is 20, and the sensitivity is 0.1 A/V. Thesystem can perform overpotential measurements of OER and HER catalystsonly after the CV operation is completed. An iR compensated LSV (iRcompensated linear sweep voltammetry) method is used for measuring theoverpotential, and the scanning range and the sensitivity are identicalto those of the CV operation in a previous step, but the scan rate isreduced to 0.5 mV/s, so that the overpotential of each point is enabledto reach a steady state.

After the measurement, at a low current density of 10 mA/cm², the OERoverpotential is 215 mV, and the HER overpotential is −10 mV (comparableto a platinum catalyst). At a high current density of 500 mA/cm², theOER overpotential is 292 mV, and the HER overpotential is −144 mV.Therefore, the method for electrolysis of water of the presentembodiment can effectively lower the overpotential required during waterelectrolysis, so that electric energy consumption can be effectivelyreduced.

Experimental Example 2

The six-element high entropy alloy including manganese, iron, cobalt,nickel, copper and molybdenum as main components is used as thecatalyst, and 1 M potassium hydroxide water solution is used as theelectrolyte for performing water electrolysis. Experimental steps andmeasurements identical to the experimental example 1 are performed.

For OER and HER measurements in alkaline media, a CV scanning range is 0to 1 V (vs. Hg/HgO electrode) and −0.8 to −1.5 V (vs. Hg/HgO electrode),respectively. For HER measurements in acidic media, the CV scanningrange is −0.5 to −1.5 V (vs. Hg/HgO electrode), a scan rate is 100 mV/s,a scanning cycle number is 20, and the sensitivity is 0.1 A/V. Thesystem can perform overpotential measurements of OER and HER catalystsonly after the CV operation is completed. An iR compensated LSV (iRcompensated linear sweep voltammetry) method is used for measuring theoverpotential, and the scanning range and the sensitivity are identicalto those of the CV method in a previous step, but the scan rate isreduced to 0.5 mV/s, so that the overpotential of each point is enabledto reach a steady state.

After the measurement, at a low current density of 10 mA/cm² in alkalinemedia, the OER overpotential is 201 mV, and the HER overpotential is −16mV. At a high current density of 500 mA/cm² in alkaline media, the OERoverpotential is 282 mV, and the HER overpotential is −159 mV.Therefore, the method for electrolysis of water of the presentembodiment can effectively lower the overpotential required during waterelectrolysis, so that electric energy consumption can be effectivelyreduced.

Additionally, the above five-element high entropy alloy catalyst and thesix-element high entropy alloy catalyst also exhibit excellentelectrochemical performances under the condition of using 0.5 M sulfuricacid aqueous solution as the electrolyte. For example, by using theabove five-element high entropy alloy and using the 0.5 M sulfuric acidaqueous solution as the electrolyte, under the condition of the currentdensity of 10 mA/cm², the HER overpotential is only −10 mV. By using theabove six-element high entropy alloy and using the 0.5 M sulfuric acidaqueous solution as the electrolyte, under the condition of the currentdensity of 10 mA/cm², the HER overpotential is only −15 mV.

A method for preparing a high entropy alloy catalyst will be illustratedhereafter.

FIG. 2 is a flow diagram of a method for preparing a high entropy alloycatalyst drawn according to an embodiment of the invention. Referring toFIG. 2, in a step 200, a substrate is put into an aqueous electrolytecontaining a high entropy alloy precursor. In the present embodiment,the substrate is a porous substrate, for example, a metal foam.Additionally, in the present embodiment, the aqueous electrolytecontaining a five-element high entropy alloy precursor and the aqueouselectrolyte containing a six-element high entropy alloy precursor arerespectively used, but the invention is not limited thereto. In otherembodiments, an aqueous electrolyte containing a multi-element (morethan six elements) high entropy alloy precursor can also be used.

Under the condition of using the aqueous electrolyte containing thefive-element high entropy alloy precursor, the contained five-elementhigh entropy alloy precursor may be (but not limited to) ferricchloride, cobalt chloride, nickel chloride, cupric chloride and ammoniummolybdate, and a chelating agent maybe added according to circumstances.

Under the condition of using the aqueous electrolyte containing thesix-element high entropy alloy precursor, the contained six-element highentropy alloy precursor may be (but not limited to) manganese chloride,ferric chloride, cobalt chloride, nickel chloride, cupric chloride andammonium molybdate, and the chelating agent maybe added according tocircumstances.

In a step 202, the substrate is subjected to an electroplating processso as to form the high entropy alloy catalyst on the substrate.Therefore, the substrate with the high entropy alloy catalyst formed onthe surface can be used as an anode and a cathode during waterelectrolysis. In the present embodiment, the current density of theelectroplating process is, for example, in a range of 2 to 6 A/cm².

In the present embodiment, the high entropy alloy catalyst is formedonto the substrate through the electroplating process, so thatpreparation steps of the high entropy alloy catalyst are simplified, andthe preparation cost is low. Additionally, on the basis ofcharacteristics of the electroplating process, the formed high entropyalloy catalyst can have a three-dimensional structure and thus have ahigh surface area, so that the water electrolysis efficiency can beeffectively improved.

The method for preparing a high entropy alloy catalyst will beillustrated hereafter by experimental examples.

Experimental Example 3

A nickel foam (with a pore density being 100 PPI) is put into theaqueous electrolyte containing the five-element high entropy alloyprecursor. Various high entropy alloy precursors in the aqueouselectrolyte are respectively ferric chloride (0.3 M), cobalt chloride(0.2 M), nickel chloride (0.5 M), cupric chloride (0.005 M) and ammoniummolybdate (0.045 M), and additionally, sodium citrate (0.4 M) is addedto be used as the chelating agent.

The pH value of the electrolyte is regulated to 9 by ammonium hydroxide.A two-electrode electroplating method is used for performing pulseelectroplating, wherein a current density is 4 A/cm², an electroplatingcycle number is 3000, the electroplating on time is 0.2 second, and thecurrent off time is 0.8 second. After the electroplating is completed,deionized water and acetone are used for cleaning a test block.

FIG. 3 is an SEM image of a five-element high entropy alloy catalyst ofthe experimental example 3. FIG. 4 is a TEM image of the five-elementhigh entropy alloy catalyst of the experimental example 3. From FIG. 3,we can clearly see that the formed five-element high entropy alloycatalyst has a great number of dendrite structures. Additionally, asshown in FIG. 4, the formed five-element high entropy alloy catalyst isof an FCC (face-centered cubic packing) structure, the interplanarspacing of a plane (111) is 0.209 nm, and the interplanar spacing of aplane (220) is 0.181 nm.

Experimental Example 4

A nickel foam (with a pore density being 100 PPI) is put into theaqueous electrolyte containing the six-element high entropy alloyprecursor. Various high entropy alloy precursors in the aqueouselectrolyte are respectively manganese chloride (0.4 M), ferric chloride(0.3 M), cobalt chloride (0.05 M), nickel chloride (0.5 M), cupricchloride (0.002 M) and ammonium molybdate (0.02 M), and additionally,sodium citrate (0.4 M) is added to be used as the chelating agent.

The pH value of the electrolyte is regulated to 9 by ammonium hydroxide.A two-electrode electroplating method is used for performing pulseelectroplating, wherein a current density is 4 A/cm², an electroplatingcycle number is 3000, the electroplating on time is 0.2 second, and thecurrent off time is 0.8 second. After the electroplating is completed,deionized water and acetone are used for cleaning a test block.

FIG. 5 is an SEM image of a six-element high entropy alloy catalyst ofthe experimental example 4. FIG. 6 is a TEM photo of the six-elementhigh entropy alloy catalyst of the experimental example 4. From FIG. 5,we may clearly see that the formed six-element high entropy alloy has agreat number of dendrite structures. Additionally, as shown in FIG. 6,the formed six-element high entropy alloy is of an FCC structure, theinterplanar spacing of a plane (111) is 0.208 nm, and the interplanarspacing of a plane (200) is 0.179 nm.

Although the present invention has been described with reference to theabove embodiments, the embodiments are not intended to limit the presentinvention. Any person skilled in the art may make variations andimprovements without departing from the spirit and scope of the presentinvention. Therefore, the protection scope of the present inventionshould be subject to the appended claims.

What is claimed is:
 1. A method for preparing a catalyst forelectrolysis of water, comprising: placing a substrate in an aqueouselectrolyte containing a high entropy alloy precursor, wherein the highentropy alloy precursor comprises manganese chloride, ferric chloride,cobalt chloride, nickel chloride, cupric chloride and ammoniummolybdate; and performing an electroplating process on the substrate toform a high entropy alloy catalyst on the substrate, wherein in the highentropy alloy catalyst, based on a total mole number of the high entropyalloy catalyst, a content of each of manganese, iron, cobalt, nickel,copper and the molybdenum is in a range of 5 to 35 at. %.
 2. The methodfor preparing a catalyst for electrolysis of water according to claim 1,wherein a current density of the electroplating process is in a range of2 to 6 A/cm².
 3. The method for preparing a catalyst for electrolysis ofwater according to claim 1, wherein the substrate comprises a poroussubstrate.